A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Abstract

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.